Modern semiconductor computing systems have often utilized sets of individually packaged semiconductor dice mounted and interconnected on a circuit board. More recent designs have eliminated the numerous individual die packages in favor of a single package capable of housing several bare dice. The advantages of such systems are greater computing power per unit area of circuit board, and lower packaging cost. FIG. 1 shows a portion of an example of such a conventional multi-dice package, illustrated in a cross-sectional view. The dice are initially formed in large groups on wafers. By cutting the wafer, the dice are individually segregated. The encapsulation device is typically molded plastic and has a chamber portion comprised of a plurality of die chambers 5. Each chamber has at least one beveled edge 10. A bare die is inserted by hand into a chamber 5 with the circuit side touching the beveled edge 10 The beveled edge 10 thus serves as a guide for the insertion of the bare die. However, since the circuit slides across the bevel, the circuitry may be damaged during insertion. The bare die is retained by a spring-retaining and contact assembly 15 located at the bottom of the chamber 5. The retaining and contact assembly 15 holds the bare die in position in the encapsulation device, with a spring portion 20 electrically contacting the bare die.
A rigid foot portion 25 is provided for contacting a circuit board onto which the encapsulation device is mounted. Due to the rigidity of the foot portion 25 and inherent bowing of many circuit boards, the failure rate of electrical contact between the bare dice and the board is typically high. At times the failure rate runs as high as 80%.
Heat dissipation is a persistent problem in the packaging systems of virtually all semiconductor devices, including the encapsulation device shown in FIG. 1. Long-term exposure to excessive temperatures may impede the operation of a die or lead to an electrical failure. There are various approaches available to lessen the problem of die heating that involve either redesigning the die circuitry or modifying the encapsulation device. For instance, designing circuits to operate at lower voltage levels may provide a partial solution. However, lower operating voltages may not be possible for a given die.
Alternatively, certain features may be incorporated into the encapsulation device itself to improve heat transfer. The encapsulation device shown in FIG. 1 provides few pathways for the transfer of heat from the dice. This is due to the relatively small amount of physical contact between the encapsulation device and the dice and to the less than optimal thermoconductivity for molded plastic. Although there will be some minimal amount of natural convective heat transfer between the dice and the ambient, the amount is of little consequence. Further, radiative heat transfer does not ordinarily play a significant role because the temperatures required for significant radiative heat transfer from the dice are normally higher than the maximum permissible operating temperature of the dice.
To improve heat transfer from the dice, a cap, of the type to be described below, may be placed on the encapsulation device shown in FIG. 1, and may be modified in several ways, depending on the heat output of the dice. For a relatively low heat output combination of dice, the cap may be made of metal to improve conduction from the die chambers 5. If additional heat transfer capacity is necessary, the cap may be fitted with slots, and possibly a forced air supply, to improve convection. The cap may be also be provided with fins to improve both the conduction and convection. The addition of fins, slots, and fans increases the complexity of the device, and space limitations may rule out their use. Furthermore, even with the aforementioned modifications, the predominant heat transfer mechanism will continue to be convection, which is less efficient than conduction.
In addition to these problems, solid caps secured over the chamber portion of the encapsulation device may not retain the dice in the correct position and often are a cause of dice damage subsequent to encapsulation of the dice.
Thus, a need exists for an encapsulation device for bare dice which provides reliable electrical contact between the dice and a mounting board and a need exists for a method for safely inserting the bare dice into the encapsulation device. In addition, there is a need to provide position retainment of the bare dice within the encapsulation device without fear of dice damage following encapsulation, and a need to incorporate conductive heat dissipation features into the encapsulation device without the need for space limiting heat sinks or circuit design changes to accommodate lower voltages.